Method and apparatus for performing versioning for loop, method and apparatus for collecting array range check information in basic blocks, method for modifying array range check information, method for optimizing array range checks, method for generating codes for array range checks, method and apparatus for eliminating redundant array range checks, method for selecting array range checks, method for modifying array range checks, method for collecting array range checks, and method for determing handling of array range checks

Abstract

Java language is, as its specification, capable of detecting an access exceeding an array range, and when there is no user-defined exception handler, moving control to an invoked method after getting out of a method in which an exception occurred, or when there is a user-defined exception handler, moving the process to the exception handler. Accordingly, an array range check is essential since occurrence of an exception may be described as a correct operation. However, an array range check slows execution speed compared with a language which does not require it. In an actual program, there is an array access to ensure that there is no access exceeding a range, and thus elimination of such redundant range checks greatly contributes to improved performance, and in addition, brings about an effect of expanding the range of optimization from the viewpoint of ensuring order of execution between occurrence of an exception and a process with a side effect such as an assignment of a value to an array.

Description

[0001] 1. Technical Field[0002] The present invention relates to a compiler, more particularly to a method for eliminating or optimizing an array range check in a compiler. An array range check is a check on whether an array access in a program is exceeding its array range.[0003] 2. Prior Art[0004] Several methods exist for eliminating an array range check based on the background art.[0005] One such method is for checking the possibility of a range exceeding before a loop. (See "Elimination of Redundant Array Subscript Range Checks", Priyadarshan Kolte and Michael Wolfe, In proceedings of the ACM SIGPLAN '95 Conference on Programming Language Design and Implementation, pp. 270 to 278, June 1995, etc.)[0006] Table 1 in which 0 is the lower bound of an array and N is its size is modified to Table 2 as follows.1TABLE 1 for (i = start; i <= end; i++) a[i] = 0;[0007]2 TABLE 2 if ((start <= end) && (start < 0 .vertline..vertline. end > N-1)) { exception; / * exceed an array ran...

AI Technical Summary

Benefits of technology

[0026] To achieve the above-mentioned objects, this invention may be categorized into the following three parts. Namely, (A) a part to eliminate redundant array range checks by performing a versioning for a loop, (B) a part to optimize array range checks by performing data-flow analysis in reverse order of the program execution, and (C) a part to obtain information about array ranges already checked by performing data-flow analysis in program execution order and eliminate redundant array range checks from this information.

Problems solved by technology

Namely, this method can only apply to a language whose specification defines that it is an error to exceed a range.

Moreover, it can only apply when an array index in a loop changes monotonously.

In addition, it cannot apply in the case that an ending condition of a loop cannot be put out of the loop, for instance, when end is a global variable, etc. in the above instance and end is changed by itself in the loop or by another thread.

However, it can only apply when an array index in a loop changes monotonously.

In addition, it cannot apply in the case that an ending condition of a loop cannot be put out of the loop, for instance, when end is a global variable, etc. in the above instance and end is changed by itself in the loop or by another thread.

However, it has a disadvantage that the range to be determined as already checked is small.

However, in reality there are many cases in which a range of a variable cannot be narrowed down.

However, it has its disadvantages, namely, the range in which it can eliminate array range checks is narrow and it can only apply to a language whose specification defines that it is an error to exceed a range.

As this occurrence of an exception may be used to write a program, a program will not run correctly without performing array range checks.

It is because, in the case of (B), it is not possible to handle it if there is an instruction which causes a side effect.

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